1. Field of the Invention
The present invention relates to a feedforward amplifier, which is one of the leading devices of linearization systems for mobile communications base stations, and a method of improving the performance thereof, and more particularly, to a feedforward amplifier using imperfect cancellation of a main signal and a method of improving the performance thereof, in which by including a predetermined amount of a main signal in an error signal that is input to an error signal cancellation loop of the feedforward amplifier, more error components of a final output signal are removed such that the linearity and efficiency of the final output signal improve.
2. Description of the Related Art
In long distance telecommunications such as satellite communications and mobile communications, a signal cannot be delivered in one direct transmission. Hence, relay stations or base stations are located at predetermined intervals so that a signal can be amplified and transmitted again. A power amplifier used in the relay stations or base stations for this operates in a saturation region having a non-linear characteristic in order to obtain a maximum output. However, in this case, when multiple carriers (multi-carriers) are input to the power amplifier, these multiple carriers cause intermodulation distortion (IMD) such that the performance of the amplifier is greatly degraded. The intermodulation distortion component is generated by interactions between two carrier waves when two or more carriers are provided and pass a non-linear device.
It has been known that removal of these intermodulation distortion components with existing filters is very difficult and these components also work as a kind of noise greatly affecting the quality of calls. In order to reduce the non-linear characteristic of the power amplifier, a variety of methods have been suggested and used. One of the leading methods is a linearization method using feedforward. Feedforward is a method by which the characteristics of a system improve by providing a part of input to an output end to modify the output result, and linearization is to convert a device or a signal having a nonlinear characteristic to one having a linear characteristic.
Meanwhile, a linear amplifier is a kind of amplifier for providing power to a load. Linear amplifiers are classified as low frequency power amplifiers and high frequency power amplifiers. In a power amplifier, it is important to provide power efficiently with less distortion.
In ordinary feedforward linearization, only the intermodulation distortion component is extracted from a signal intermodulation-distorted with a carrier wave obtained from the output of a power amplifier, and the extracted intermodulation distortion component is again coupled, with the phase of the component inversed, to the output of the amplifier. By doing so, the intermodulation distortion signal is canceled and so that a carrier to intermodulation ratio, that is, the ratio of the power of the carrier to the power of the intermodulation distortion signal, improves.
FIG. 1 is a schematic diagram of the structure of an ordinary feedforward amplifier. Referring to FIG. 1, the feedforward amplifier basically includes two cancellation loops 10 and 20. The cancellation loop 10 is a main signal cancellation loop and the cancellation loop 20 is an error signal cancellation loop. Each loop 10 and 20 comprises a power amplifier 12 and 22, a powercoupler 14 and 24, a vector modulator 16 and 26, and a delay line 18 and 28.
The main signal cancellation loop 10 of FIG. 1 extracts only an error component of the main amplifier 12, using the difference of the input signal of the entire system and a non-linear signal amplified by the main amplifier 12. The error signal cancellation loop 20 amplifies the error component, which is provided from the main signal cancellation loop 10, through an error amplifier 22, inverts the phase of the error component, and couples the result with the output signal of the main amplifier 12 so that only the error component of the output signal of the main amplifier 12 is removed.
As described above, after being amplified by the main amplifier 12 through the two loops 10 and 20, the non-linear signal becomes linearized. It has been known that this feedforward amplifier 1 has good performance in linearity improvement, covers a wide band at the same time, and has good stability.
The prior art feedforward amplifier 1 as shown in FIG. 1 usually uses an error signal with a perfectly canceled main signal, which is schematically shown at the top of FIG. 3. As shown at the top of FIG. 3, a control method for generating an error signal with a perfectly canceled main signal is realized by the main signal cancellation loop 10 and the error signal cancellation loop 20 detecting and controlling different signals, respectively, as shown in FIG. 2. That is, in the adaptive control method of the prior art feedforward amplifier 1, the detection unit 1 detects an error signal, the control unit 1 controls the main signal cancellation loop 10, the detection unit 2 detects the error component of the final output signal, and the control unit 2 controls the error signal cancellation loop 20, as shown in FIG. 2. Here, the main signal cancellation loop 10 is adaptively controlled so that the component of the main signal in the detected error signal is minimized. In this way, the main signal in the error signal is perfectly canceled.
However, since most systems use a modulated signal as an input signal, an error signal with an imperfectly canceled main signal, instead of an error signal with a perfectly canceled main signal, can show optimal performance.
To solve the above problems, it is an object of the present invention to provide a feedforward amplifier and a method of improving the performance thereof, in which a predetermined amount of a main signal is made to be included in an error signal input to the error signal cancellation loop of a feedforward amplifier, considering that in the feedforward amplifier, an error signal with an imperfectly canceled main signal, instead of an error signal with a perfectly canceled main signal, can show optimal performance.
That is, it is an object of the present invention to provide a feedforward amplifier using imperfect cancellation of a main signal and a method of improving the performance thereof, in which by including a predetermined amount of a main signal in an error signal that is input to an error signal cancellation loop of the feedforward amplifier, more error components of a final output signal are removed such that the linearity and efficiency of the final output signal improve.
According to an aspect of the present invention, there is provided a feedforward amplifier comprising a main signal cancellation loop which comprises a main amplifier and a vector modulator and extracts an error component of the main amplifier; an error signal cancellation loop which comprises an error amplifier and a vector modulator and removes the error component in the output signal of the main amplifier; a final error signal detection unit which detects an error component of a final output signal of the feedforward amplifier; a main signal cancellation loop control unit which controls the main signal cancellation loop based on the error component detected by the final error signal detection unit; and an error signal cancellation loop control unit which controls the error signal cancellation loop based on the error component detected by the final error signal detection unit, wherein the error signal generated by the main signal cancellation loop is optimized in the direction linearizing the final output signal and the main signal cancellation loop is adaptively controlled by the main signal cancellation loop control unit.
It is preferable that the error signal output from the main signal cancellation loop has an imperfectly canceled main signal.
It is preferable that the output signal of the main signal cancellation loop control unit is input to the vector modulator of the main signal cancellation loop, and the output signal of the error signal cancellation loop control unit is input to the vector modulator of the error signal cancellation loop.
According to another aspect of the present invention, there is provided a method for improving the performance of a feedforward amplifier comprising a main signal cancellation loop and an error signal cancellation loop, the method comprising detecting the error component of a final output signal of the feedforward amplifier; and controlling the main signal cancellation loop and the error signal cancellation loop at the same time using the detected error component.
It is preferable that the method further comprises, before controlling the loops, detecting the error signal of the main signal cancellation loop and using the error signal, controlling the main signal cancellation loop; and detecting the error component of a final output signal of the feedforward amplifier and controlling the error signal cancellation loop.